The present disclosure relates to an exhaust gas purification catalyst.
The exhaust gas purification catalyst (three-way catalyst) purifying HC (hydrocarbon), CO (carbon monoxide), and NOx (nitrogen oxide) in engine exhaust gas generally contains, e.g., activated alumina and Ce-containing oxide in addition to catalytic metal such as Pt, Pd, and Rh. Since the activated alumina has high heat resistance and a large specific surface area, the activated alumina has been conventionally used as a support of the catalytic metal, and can maintain a state in which the catalytic metal is supported so as to be relatively highly dispersed even if the catalyst is exposed to high-temperature exhaust gas. The Ce-containing oxide stores and releases oxygen depending on fluctuation in oxygen concentration of exhaust gas, and therefore has been used as an oxygen storing/releasing material expanding an A/F window (an air-fuel ratio range in which the catalyst can simultaneously purify HC, CO, and NOx).
CeZr-based mixed oxide containing Ce and Zr has been known as the Ce-containing oxide. The CeZr-based mixed oxide has higher heat resistance as compared to that of ceria. In addition, since Zr dissolution causes crystal strain and an increase in an oxygen defect, an oxygen storing/releasing capacity comparable to that of ceria is provided. Further, CeZr-based mixed oxide has been known, in which rare-earth metal such as Nd, Y, and La is solid-dissolved.
The CeZr-based mixed oxide having a particle size (diameter) of about 1 μm has been generally used for the catalyst. The catalytic metal is physically supported on surfaces of CeZr-based mixed oxide particles by, e.g., evaporation to dryness or impregnation, thereby increasing an oxygen storing/releasing amount. This is because the catalytic metal is a medium accelerating the storage/release of oxygen by the CeZr-based mixed oxide. In such a case, the oxygen storing/releasing amount of the CeZr-based mixed oxide is also significantly low as compared to a theoretical value. This is because only surface portions of the particles having the particle size of about 1 μm are used for the oxygen storage.
As an oxygen storing/releasing material for which the foregoing point is improved, catalytic-metal-doped CeZr-based mixed oxide has been known, in which catalytic metal is solid-dissolved in CeZr-based mixed oxide (see Japanese Patent Publication No. 2004-174490). The term “doped” means that the catalytic metal is solid-dissolved in the mixed oxide, and is placed at crystal lattice points of the mixed oxide or between atoms of the mixed oxide. In the catalytic-metal-doped CeZr-based mixed oxide, the oxygen storing/releasing amount and an oxygen storing/releasing speed are dramatically improved as compared to those of CeZr-based mixed oxide which is not doped with the catalytic metal. This is because some of the catalytic metal particles are inside a mixed oxide particle, and accelerate movement of oxygen atoms inside the mixed oxide particle. That is, this is because not only the surface portion of the particle but also the inside of the particle are used for the storage/release of oxygen.
According to the catalytic-metal-doped mixed oxide, there are advantages that not only oxygen storing/releasing performance is improved, but also a catalytic metal amount of the catalyst can be significantly reduced. The catalytic-metal-doped CeZr-based mixed oxide is not limited to the use for the three-way catalyst, but can be used for, e.g., a lean NOx catalyst purifying NOx exhausted from a lean-combustion gasoline engine, and a PM (particulate matter) combustion catalyst efficiently oxidizing and combusting PMs trapped by a diesel particulate filter.
However, in the catalytic-metal-doped CeZr-based mixed oxide particles, all of the catalytic metal particles are not present on the particle surfaces, and a part of the catalytic metal particles is merely exposed on the particle surfaces. Thus, it is less likely that the catalytic metal and exhaust gas contact each other, and improvement of exhaust gas purification performance by doping the catalytic metal is limited.
On the other hand, the applicant of the present disclosure has proposed one solution in Japanese Patent Publication No. 2006-334490. In such a solution, the applicant has proposed that, on surfaces of first mixed oxide (CeZr-based mixed oxide) particles which are not doped with catalytic metal, second catalytic-metal-doped mixed oxide (CeZr-based mixed oxide) particles having a particle size smaller than that of the first mixed oxide particle are supported. According to such a catalytic particle, a catalytic metal concentration on a catalytic particle surface is higher than that inside the catalytic particle, and therefore relatively high catalytic activity can be achieved even with a small catalytic metal amount.